1. Field of the Invention
This invention relates to digital switching systems and more particularly to the switching of digital signals through a PAM (pulse amplitude modulated) bus network.
2. Description of the Prior Art
A well known prior art private branch exchange uses a PAM bus network to establish voice frequency connections among pluralities of lines and trunks. Some of the different kinds of connection paths that can be established in such a PBX are illustrated in U.S. Pat. No. 3,934,091 entitled "Automatic Gain Compensation in Time Division Switching System" issued to James R. Stidham on Jan. 20, 1976. Different amplification or gain factors are present on line-to-line, line-to-trunk and trunk-to-line connections. The gain that is provided when a voice connection is made between two lines appearing in the same line carrier (where they are served by the same line summing amplifier input for the PAM bus) is somewhat different than the gain provided when the connection is made between two lines that appear in different line carriers (where each line is served by a separate input of the line summing amplifier).
In that prior-art system, the voice ports assigned to lines and trunks are sampled by the PAM bus at approximately twice the Nyquist rate, i.e., at approximately 16.2 kHz, so that simpler passive filters can be employed than would be required if these ports were sampled at the conventional Nyquist rate. Since this sampling rate exceeds the conventional data modem transmission rates of 1.2, 2.4, 4.8 and 9.6 kilobaud, the possibility presents itself of applying the digital data stream directly to the PAM bus, i.e., without employing a modem to first have the data stream modulate a voice-band carrier signal and to then apply the composite signal to the PAM bus. While modems might still be required to perform the usual modulation, wave shaping, and equalization processes when the digital data transmission path extends beyond the PBX, if modems could be eliminated on intra-PBX calls the total number of modems required at a PBX could be reduced by the ratio that intra-PBX data traffic bears to total data traffic.
When digital data signals are received it is generally necessary to determine whether the received signal level corresponds to a "1", to a "0", or to some other predetermined signal level. For example, if a positive voltage of some magnitude is intended to signify a "1" and a negative voltage is intended to signify a "0", the zero-voltage level is the obvious threshold or "slicing" level upon which to base the decision of what the signal means. In the above-mentioned PAM bus system, however, the DC output level delivered by the PAM bus to the called port circuit varies depending upon the particular connection path that may have been established through the network on the particular call. This variation in DC level is of no consequence on a voice connection because an analog voice signal has no DC component. The DC offset in the PAM bus network is present because each of the buffer amplifiers employed in the PAM bus system contains a deliberate DC offset (approximately 50 millivolts) for the purpose of mitigating crossover distortion. Crossover distortion is caused when an analog amplifier employs separate transistor amplifier sections for handling positive and negative input signals. When the input signal changes polarity a transition is made from one amplifier section to the other. To avoid having this transition take place at very low signal levels, which might lead to uncertainty as to which (or both) sections would be operative, a deliberate DC offset is employed so that one set of amplifiers will handle all low level signals without regard to polarity.
While the nominal DC offset level produces no ill-effects in voice communications, the normal variation in the nominal level exhibited by different combinations of such summing amplifiers means that the ultimate DC voltage output level cannot accurately be predicted. When additional stages of switching, each including a buffer amplifier, are added to the aforementioned type of switching system to accommodate greater numbers of lines and trunks, the gain (and output level) on each voice connection from an original source port to a final destination port will also vary according to the number of summing amplifiers included in the connection. Accordingly, it would be advantageous to establish a uniform DC output level on the distribution bus so that a predictable slicing level for digital signals could be achieved regardless of the particular connection path that may be taken through the PAM bus network on any call.